Date: 02/27/2002 at 13:02:40
From: Doctor Peterson
Subject: Re: Bases in Math
Hi, Billy.
Have you seen our FAQ on Number Bases?
http://mathforum.org/dr.math/faq/faq.bases.html
There are several basic explanations of bases there. I'll give a brief
introduction.
You are given the number 13 in base ten; this means 1 ten and 3 ones.
You want to write it in base five, as some number of fives and some
number of ones. (If the number were big enough, you might also need
25's, 125's, and so on, just as in base ten you might need hundreds or
thousands.)
So here is 13:
___10___ 1 1 1
/ \
oooooooooo o o o
\________/ \___/
1 3
ten ones
Let's group it by 5's rather than 10's:
_5_ _5_ 1 1 1
/ \ / \
ooooo ooooo o o o
\_________/ \___/
2 3
fives ones
So we have 2 fives and 3 ones; in base five, we write this as
23 (base 5).
Let's try a bigger number: 68 (base ten). We can group it in fives by
just dividing by 5, rather than drawing pictures:
68 / 5 = 13 r 3
So we have 13 fives and 3 ones.
But we're not done, because in base five we can't have any digits
greater than 4 (just as in base ten we have no digits greater than 9).
So now we have to group our 13 fives into groups of five fives.
Divide again:
13 / 5 = 2 r 3
This tells us that 13 is 2 fives and 3 ones; so 13 FIVES is
2 twenty-fives and 3 fives. Our number is then
68 (base ten) = 2 (25's) + 3 (5's) + 3 (1's)
which we write as 233.
_25 _25 _5_ _5_ _5_ 1 1 1
/ \ / \ / \ / \ / \
ooooo ooooo ooooo ooooo ooooo o o o
ooooo ooooo
ooooo ooooo
ooooo ooooo
ooooo ooooo
\_________/ \_______________/ \___/
2 3 3
twenty-fives fives ones
Check it out:
2x25 + 3x5 + 3x1 = 50 + 15 + 3 = 68
If you aren't sure what I mean by bases in the first place, try
reading this explanation referred to in our FAQ, then come back here
and see if you can follow what I said:
Counting: Base 6, 12, 16
http://mathforum.org/dr.math/problems/asplund5.31.96.html
- Doctor Peterson, The Math Forum
http://mathforum.org/dr.math/